In fabrication processes of semiconductor devices, large amounts of wastewater containing anticorrosives for copper are discharged from CMP steps for performing surface polishing of copper when copper wiring is installed. Therefore, treatment of the wastewater is required.
Among anticorrosives for copper, in particular, an azole-type anticorrosive for copper has an excellent anticorrosive effect. However, the azole-type anticorrosive for copper has a chemically stable structure and is not easily biodegraded. Thus, conventionally, in treating wastewater containing an azole-type anticorrosive for copper discharged from the steps, the azole-type anticorrosive for copper is decomposed using an oxidizing agent having high oxidizing power, such as ozone, ultraviolet, or hydrogen peroxide, or by an advanced oxidation process in which these oxidizing agents are combined, and then treated water is discharged or collected.
However, as described above, since the azole-type anticorrosive for copper is chemically stable, even in use of an oxidizing agent having high oxidizing power, such as ozone, addition of a large amount thereof is required for oxidative decomposition of the azole-type anticorrosive for copper, thus posing a big problem in terms of cost. In particular, in recent years, with the increase in the degree of integration in semiconductor devices, the number of fine polishing steps has been increasing, and along with this, the amount of polishing wastewater discharged has been increasing. Therefore, the increase in cost due to an increase in the capacity of wastewater treatment equipment has become a problem.
A method has been proposed, in which the pH of wastewater containing benzotriazole serving as an anticorrosive for copper is adjusted to a weakly acidic state, the treated liquid is allowed to react with an oxidizing agent in an oxidative decomposition tank, the treated liquid is further adjusted to pH 10 or more in an alkalinity adjustment tank, and the resulting copper hydroxide is subjected to solid-liquid separation (Patent Literature 1). However, even in this method, a large amount of the oxidizing agent is required to decompose a chemically stable azole-type anticorrosive for copper. Furthermore, in particular, in the case where ozone is used as an oxidizing agent, self-decomposition of ozone is suppressed under an acidic pH condition, the amount of OH radicals generated in the decomposition of ozone and having higher oxidizing power is decreased, and thus oxidizing power is also decreased, which is a problem.
Furthermore, wastewater from CMP steps contains, in addition to the azole-type anticorrosive for copper, abrasive particles (suspended solids), such as colloidal silica, used in the CMP steps. Prior to treatment with an oxidizing agent, if solid-liquid separation treatment, such as flocculation, sedimentation, or filtration, is not performed as a pretreatment, the oxidizing agent is wasted in decomposing the suspended solids. As a result, the effective utilization efficiency of the oxidizing agent decreases, and a decomposition effect that corresponds to the amount of the oxidizing agent added is not sufficiently exhibited, which is also a problem. Furthermore, in the case where the suspended solids flow in the oxidative decomposition tank, there is a possibility that the suspended solids may be deposited in the tank or clog a diffuser tube placed in the tank for injecting ozone, which may inhibit oxidation treatment.    Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2002-35773